CENTRAL RIDGE OFF NEWFOUNDLAND: LITTLE EXPLORED, POTENTIAL LARGE

Neil R. de Silva Canada-Newfoundland Offshore Petroleum Board St. John's, Newf. The Central ridge on the northeastern Grand Banks off Newfoundland represents a large area with known hydrocarbon accumulations and the potential for giant fields. It covers some 17,000 sq km with water less than 400 m deep. It forms a positive tectonic element that separates the prolific Jeanne d'Arc basin to the west from the Flemish Pass basin to the east. To the north it is terminated by the Orphan

Oct. 25, 1993

4 min read

Neil R. de Silva
Canada-Newfoundland Offshore Petroleum Board
St. John's, Newf.

The Central ridge on the northeastern Grand Banks off Newfoundland represents a large area with known hydrocarbon accumulations and the potential for giant fields.

It covers some 17,000 sq km with water less than 400 m deep. It forms a positive tectonic element that separates the prolific Jeanne d'Arc basin to the west from the Flemish Pass basin to the east. To the north it is terminated by the Orphan basin, and to the south it merges with highs related to the Avalon uplift (Fig. 1).

The first major hydrocarbon discovery on the Newfoundland Grand Banks is giant Hibernia field in the Jeanne d'Arc basin. Hibernia field, discovered in 1979, has reserves of 666 million bbl and is due onstream in 1997.

Since Hibernia, 14 other discoveries have been made on the Grand Banks, with three on the Central ridge (Table 1). Oil was first discovered on Central Ridge in 1980 with the Mobil et al. South Tempest G-88 well. In 1982 gas was discovered with the Mobil et al. North Dana I-43 well 30 km northeast of the earlier discovery.

In 1983 gas and condensate were discovered with the Husky-Bow Valley et al. Trave E-87 well 20 km south of the South Tempest well. These discoveries are held under significant discovery licenses and an additional 2,400 sq km are held under exploration licenses.

HISTORY, POTENTIAL

The Central ridge emerged as a positive structural feature in Late Jurassic and Early Cretaceous times as a result of tectonics related to the rifting of Iberia and North America.

This interval saw the formation of a large, northwest trending structural high called the Avalon uplift on the southern Grand Banks and a number of north-south trending horsts and grabens elsewhere This led to the formation of a number of deep basins with restricted circulation conducive to the accumulation of organic-rich sediments.

Shallow marine and fluvial sandstones that make up the productive Jeanne d'Arc and Hibernia reservoirs in the Jeanne d'Arc basin were deposited at this time.

RESERVOIR-SOURCE-TRAPS

The late Jurassic (Kimmeridgian) stratigraphy of the Central ridge is shown in Fig. 2.

Fig. 2A is a northwest-southeast cross-section of five wells, and Fig. 2B is a northeast-southwest cross-section of three wells. Their locations are shown on Fig. 1. As seen in the northwest-southeast cross-section, the Kimmeridgian interval thins to the south due to erosion and is absent on structural highs associated with the Avalon uplift.

All the wells show sand-prone intervals in the Kimmeridgian interval. These sandstones are informally referred to as "Tempest sandstones" and range from very fine to very coarse with porosities varying from poor to good. They are thickest in the South Tempest G-88 well and occur interrelated with shales and marlstones with rich source rocks occurring above, below or sandwiched between the sandstone internals.

These sandstones are believed to represent turbidities deposited in deep restricted basins (Fig. 4). The thick, hydrocarbon bearing sandstones in the South Tempest well may represent a fairway for turbidite deposition and detailed seismic interpretation with emphasis on sequence stratigraphy may delineate more plays and fairways in this promising area.

Kimmeridgian age source rocks were encountered in all the wells (Fig. 2) except in Golconda C-64 well, where they have been eroded. These source rocks have total organic carbon contents approaching 10% and have excellent hydrocarbon generating potential. They are mostly within or close to the oil generating window.

In addition to the Kimmeridgian, good source rocks occur in the Lower Tertiary interval and where buried deeply could have produced hydrocarbons.

The Central ridge contains a variety of structural traps formed during Lower Cretaceous and Jurassic rifting. These include tilted fault blocks, drape over basement highs, and possible salt and inversion structures (Fig. 4). Stratigraphic traps occur where Early Cretaceous and Late Jurassic sandstones subcrop beneath the Base Tertiary unconformity like in the Trave E-87 well.

POTENTIAL

The Central ridge has a high hydrocarbon potential owing to the fortunate coincidence of reservoirs, mature source rocks, and traps.

Three hydrocarbon discoveries have been made, and a large area with water less than 400 m deep is available for exploration. Several large structural closures with potential for hydrocarbon accumulations are present.

Several seismic programs have been conducted in the area, and seismic sections and well data released to the public are readily available at cost of reproduction from., the Canada-Newfoundland Offshore Petroleum Board.

A renewed interpretation and exploration effort here could lead to recognition of plays that could harbor giant hydrocarbon volumes.